Nearly all visceral organs have striking left-right (LR) asymmetry in their anatomy and placement within the body cavities. Hierarchies of unilaterally expressed genes have been shown to affect LR patterning; but neither the initial source of asymmetry nor the mechanism that invariantly orients the LR axis (including asymmetric gene expression) are understood. The proposed research will test the model that gap junctional communication (GJC) acts prior to organ formation to orient bilateral asymmetry of the entire embryo. Although GJC may act in several ways, one of the simplest is that LR determinants might be partitioned through gap junctions so that individual cells would know whether they lie to the left or right of the embryo's midline. Evidence in support of this model is presented. First, heterotaxia involving mirror-image reversals of heart, gut and gall bladder can be induced by treating Xenopus embryos with drugs that block or open gap junction channels. Greatest sensitivity occurs during cleavage and gastrulation, well before organogenesis. Second, genetic modulation of the spatial pattern of junctional communication also induces heterotaxia. Dorsal cells of a cleavage stage embryo are normally connected by gap junctions, whereas ventral cells are relatively uncoupled. Either the introduction of junctions ventrally or their disruption dorsally (but not the reverse) cause heterotaxia without affecting anteroposterior or dorsoventral pattern. Third, these genetic and pharmacologic manipulations also affect the normal left-sided expression of a Xenopus nodal homologue (XNR-1), inducing bilateral, right sided, or absent expression. Fourth, the model appears to apply to early streak stage chick embryos as well. While the data demonstrate that GJC is crucial for orienting LR asymmetry, they do not provide a mechanism. Thus, the objectives are to: 1) identify the connexins involved and their location in bothXenopus and chicks, 2) determine if junctional conductance acts upstream or downstream of the well-characterized LR patterning of Hensen's node in chicks, 3) explore the possibility that endogenous currents electrophorese molecules asymmetrically in the embryo and use a pharmacologic approach to identify ion channels that may establish such currents, and 4) begin to identify the molecule(s) partitioned through gap junctions. Addressing the mechanism responsible to orient LR asymmetry should contribute to our understanding of congenital laterality disorders such as situs inversus, poly- or asplenia and heart defects such as dextrocardia and transposition of the aorta and pulmonary artery.